1. Field of the Invention
This invention relates to an apparatus for after-treating polyolefin powder. More particularly, it relates to a horizontal and cylindrically shaped apparatus for after-treating polyolefin powder, which is suitable for carrying out at a low speed, such after-treatments as devolatilization of and deactivation of catalyst residue or the like in sticky polyolefin powder after completion of polymerization.
On account of the recent advanced high activity catalysts for olefin polymerization, the amount of use of a catalyst is reduced and operation for removing a catalyst after completion of polymerization has become omissible. Thus gas phase polymerization process and bulk polymerization in which after-treatment process is simple, is being watched with keen interest. However, even when the removal of catalyst residue is considered unnecessary because the amount is small, activity is still remaining in the catalyst residue included in the polyolefin powder after completion of polymerization, and hence it is necessary to stabilize the catalyst residue by deactivation. Further, a very small amount of a monomer or a solvent which has entered a polymerization vessel together with a monomer or a catalyst at the time of their addition is still remaining even in the polyolefin powder obtained in a gas phase polymerization or a bulk polymerization process, and devolatilization of these materials is necessary. Accordingly, even when removal of catalyst residue is unnecessary, after-treatment is required for the purpose of deactivation of the catalyst and devolatilization of a monomer and the like.
2. Description of the Prior Art
As conventional after-treatment apparatuses for polyolefin powder, following driers have been commonly used.
(i) A type of driers which receive heat from hot wind, such as flush drier, fluidized layer drier and the like.
(ii) Horizontal double axial paddle type driers provided with an indirect heating apparatus such as jacket, paddle and the like.
(iii) Middle or high speed revolving type, horizontal uniaxial driers (number of revolution of 200 r.p.m. or more) in which both the indirect heating with a jacket and the direct heating by hot wind are simultaneously used.
However, when the powder of a copolymer e.g. propylene-ethylene copolymer, propylene-butene copolymer, having a high comonomer content is subject to an after-treatment by using such a drier as above-mentioned, there have been following problems on account of the stickiness and extremely low fluidity of the powder.
(i) When a type of a drier receiving heat from hot wind is used, fluidization of sticky polyolefin powder by hot wind is difficult, and both devolatilization and deactivation of catalyst have been insufficient.
(ii) When a horizontal double axial paddle type drier is used, sticky polyolefin powder does not flow depending upon a kind of paddle blades. Further, since polyolefin powder severely adheres to the heating surface of paddles and is rotated together with the paddle, neither stirring effect nor transportation of the olefin powder takes place. Further, in an advanced state of adhesion of polyolefin powder, a motor for driving paddle blades is often over- loaded.
(iii) When a middle or high speed revolving horizontal type uniaxial drier is used, although temperature rises even with sticky polyolefin powder, since the holding amount of powder insides the drier is small and the powder passes through the drier within a short period of time, it requires many driers to insure a holding time necessary and satisfactory for devolatilization as well as deactivation of catalyst residue. Thus practical use in commercial scale was extremely uneconomical. Further, even in case of horizontal uniaxial driers, those of double ribbon type or flat paddle blade type can hardly be used economically in commercial scale due to the severe adhesion of powder to stirring blades or to a revolving shaft.
An object of the present invention is to provide an apparatus for after-treatment of polyolefin powder, which is not only free from such drawbacks above-mentioned conventional driers but also is provided with superior properties required for an after-treatment apparatus for polyolefin powder, particularly, such superior properties as capability of fully conducting deactivation of catalyst included in powder, devolatilization of very small amount of solvent, monomer, oligomer, volatile catalyst decomposition product, etc. even in case of sticky polyolefin powder.